The present invention relates to ink jet printing apparatus and is concerned, more particularly, with a valve for the printing apparatus maintenance system for a printhead in such apparatus.
An ink jet printer of the so-called "drop-on-demand" type has at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink may be contained in a plurality of channels and energy pulses are used to cause the droplets of ink to be expelled, as required, from orifices at the ends of the channels.
In a thermal ink jet printer, the energy pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable by current pulses to heat and vaporize ink in the channels. As a vapor bubble grows in any one of the channels, ink bulges from the channel orifice until the current pulse has ceased and the bubble begins to collapse. At that stage, the ink within the channel retracts and separates from the bulging ink which forms a droplet moving in a direction away from the channel and towards the recording medium. The channel is then re-filled by capillary action, which in turn draws ink from a supply container. Operation of a thermal ink jet printer is described in, for example, U.S. Pat. No. 4,849,774.
One particular form of thermal ink jet printer is described in U.S. Pat. No. 4,638,337. That printer is of the carriage type and has a plurality of printheads, each with its own ink supply cartridge, mounted on a reciprocating carriage. The channel orifices in each printhead are aligned perpendicular to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped, perpendicular to the line of carriage movement, by a distance equal to the width of the printed swath and the carriage is then moved in the reverse direction to print another swath of information.
It has been recognized that there is a need to maintain the ink ejecting orifices of an ink jet printer, for example, by periodically cleaning the orifices when the printer is in use, and/or by capping the printhead when the printer is out of use or is idle for extended periods. The capping of the printhead is intended to prevent the ink in the printhead from drying out. There is also a need to prime a printhead before initial use, to ensure that the printhead channels are completely filled with ink and contain no contaminants or air bubbles. After much printing and at the discretion of the user, an additional but reduced volume prime may be needed to clear particles or air bubbles which cause visual print defects. Maintenance and/or priming stations for the printheads of various types of ink jet printers are described in, for example, U.S. Pat. Nos. 4,364,065; 4,855,764; 4,853,717 and 4,746,938 while the removal of gas from the ink reservoir of a printhead during printing is described in U.S. Pat. No. 4,679,059.
The priming operation involves either forcing or drawing ink through the printhead and out the printhead nozzles, while the nozzles are sealingly enclosed by a cap. This means that hoses or conduits must be provided to remove the ink removed from the printhead during the priming operation. Depending upon which priming stage the maintenance station is in, a valve must be used to close periodically the hoses and isolate the cap. Since the hoses are normally open, the valve must be opened and closed, and, therefore, a robust cost effective valve is required to effect this closure. The most widely used priming technique for thermal ink jet printers is to subject the printhead nozzles to a vacuum or negative pressure to withdraw ink from the nozzles rather than to subject the printhead to pressure to force the ink from the nozzles. Accordingly, the maintenance station for a thermal ink jet printhead generally uses a vacuum pump to suck or draw ink from the printhead nozzles. However, many valves used to close off the maintenance station caps tend to fail with use because of the accumulation of ink in crevices, corners, and the like, which dry out. This dried ink within the valves causes them to fail eventually. Ball and seat valves and flapper valves are especially vulnerable to this failure mode.